Refined spin Hamiltonian for Yb2Ti2O7 and its two competing low field states*

We present a neutron scattering study of pyrochlore Yb2Ti2O7 in a [111] magnetic field using high-quality stoichiometric single crystals. High magnetic fields (1.5 T) reveal sharp spin wave modes, which we use to further refine the magnetic Hamiltonian, offering corrections to previous Hamiltonian refinements. Low magnetic fields reveal diffuse features which suggest an unconventional ground state, and we discuss mechanisms explaining how such a spectrum can be caused by proximity to a ferromagnetic-antiferromagnetic phase boundary. We present a model which accounts for the observed spectrum, and resolves several long-standing questions concerning the ground state of Yb2Ti2O7 [1].

[1] A. Scheie, J. Kindervater, G. Sala, G. Ehlers, S. Koopayeh and C. Broholm, in preparation.